The assignee of the present invention designs and manufactures spacecraft for, inter alia, communications and broadcast services from geosynchronous orbit. Electrical power for such spacecraft is conventionally generated by one or more photovoltaic solar arrays, each solar array typically having a planar surface, once deployed into an operational on-orbit configuration, of several hundred square feet. Pressure due to solar radiation impingement on the solar array sun-facing planar surface can result in considerable force. To the extent the center of pressure of the solar radiation impingement is offset from the spacecraft center of mass, a significant disturbance torque (“solar torque imbalance”) may be developed.
As disclosed in U.S. Pat. No. 6,053,455, assigned to the assignee of the present invention, a geosynchronous satellite may be designed to minimize solar torque imbalance. This may be accomplished, referring now to FIG. 1, Detail A, with a spacecraft configuration providing for a pair of solar arrays that are disposed so as to be symmetric about the spacecraft center of mass. Thus, in the illustrated example, the solar arrays 126(1) and 126(2) are located, respectively on the north and south side of the spacecraft main body 110, and respective torques from the two solar arrays, being of opposite signs, are substantially nulled (or “balanced”) out. Alternatively, referring now to FIG. 1, Detail B, a spacecraft configuration is known wherein a solar array 126 is disposed on the north (or south) side of the spacecraft main body 110, and solar torques associated with the solar array 126 are balanced out by disposing a solar sail 129 on the opposite side of the spacecraft main body 110. To the extent there is a remaining solar torque imbalance, the residual disturbance torques may be accumulated in momentum wheels that are then unloaded periodically using thrusters, magnetic torquers, trim tabs, or solar panel angle adjustments.
Geosynchronous spacecraft may include provisions for controlling drift orbit inclination by performing periodic north-south stationkeeping (NSSK) maneuvers. Referring still to FIG. 1, NSSK maneuvers may be performed by thrusters 131. In order to reduce impingement of thruster plume onto the solar array 126 (or solar sail 129), thrusters 131 have been conventionally located proximate to an anti-earth portion of the spacecraft. In order to develop a thrust vector through the spacecraft center of mass 102 the thrusters 131 may be canted by an angle θ of 30° or more away from the north-south (N/S) direction (Y axis). As a result, efficiency of NSSK maneuvers is reduced because the thrusters produce a substantial component of thrust in the Z axis (earth/anti-earth) direction as well as the N/S direction.